The present invention relates to a circuit for eliminating interference with an IF information signal. More particularly, the invention relates to a circuit for canceling the feed thru of a local oscillation (LO) signal to an IF signal path.
In conventional IF transmission circuits the IF signal is upconverted by mixing with an LO signal to form the signal to be transmitted. The signal to be transmitted is preferably characterized by two lobes, or sidebands, centered about the LO frequency. Ideally, the LO signal does not feed thru the mixer and does not appear in the output signal However, it is common for the LO signal to not only feed through to the mixer output (as a third lobe intermediate to the sidebands), but to be the largest amplitude component of the transmitted signal. For certain applications it is desirable to limit the transmitted signal to the information component excluding extraneous components. One advantage of such a limitation is to reduce the susceptibility of transmitted signals to interception and destructive interference. Where the LO feed thru signal amplitude exceeds the amplitude of the side lobes, interception of the transmit signal may be accomplished by identifying the LO feed thru components and concentrating on the frequency range about the LO feed thru signal. Consequently, the presence of the LO feed thru signal, as a component of the transmitted signal, renders a transmitted signal significantly more susceptible to identification and interference. The present invention is directed to a method and circuitry for reducing the ability of a receiver to identify and interfere with a transmitted signal by removing the appearance of the LO signal component from the transmitted signal.
There are a number of contemporary circuits that operate to actively remove the LO signal component from the output signal. One such circuit is disclosed in an article titled Circuit for Cancelling Local Oscillator Leakage Through Mixers, by Ron Minarik, Microwave Journal, p. 182-186, July 1985. That article discloses a circuit wherein a portion of the LO signal is tapped, phase shifted and attenuated by manual adjustments so as to be of equal amplitude and opposite phase in relation to the fed thru LO signal. The phase shifted and attenuated LO signal is then combined with the output signal so as to cancel the fed thru LO signal component from the output signal.
Another contemporary circuit designed to perform a related function is set forth in the Anaren Microwave Components Catalog, 17A, p. 158 (1985). The circuit disclosed therein operates to prevent jamming of a nearby receiver by cancelling the undesired component of the transmitted signal. The circuit achieves cancellation by tapping a portion of the transmitter signal, comparing it to the received signal, and actively cancelling the transmitter signal by adjusting the amplitude and phase.
Though these and other contemporary systems provide means for canceling an undesired LO signal component from a composite signal, the method and circuitry for performing that function have limitations and cost penalties which make their use impractical for many applications. Manually adjusted compensation circuits, such as that disclosed in the article titled Circuit for Cancelling Local Oscillator Leakage Through Mixers fail to provide automatic closed loop control to consistently perform compensation without constant manual adjustment. Changes in temperature and other conditions effect LO feed thru such that the output signal must be continuously monitored and the correct compensation signal must be continuously updated. Other circuits, such as that disclosed in the Anaren Microwave Components Catalog, require utilization of expensive circuit components which render such compensation circuits unsuitable for many applications.
In view of these and other deficiencies in contemporay compensation circuits, the present invention is intended to provide an alternate, cost effective circuit and method for canceling LO feed thru. The present invention provides a highly effective circuit wherein LO signal feed thru be reduced by as much as 60-70 dB. The cancellation signal may be adaptive to automatically respond to changes in temperature and other enviornmental conditions that will affect the nature of the fed thru signal. In addition, the circuit utilizes inexpensive available components to perform the cancellation. As described below the invention simultaneously forms its two composite signals having both IF and LO feed thru components with the LO feed thru components being canceling. The combination of the two signals eliminates LO feed thru to the output signal.